SizePecten maximus are hermaphrodite and, therefore, there is no separate male and female size range or size at maturity. Pecten maximus grows up to 15 cm and will be at least 6 cm when sexually mature.

RecessionPecten maximus normally lies recessed into slight hollows (recesses) in the seabed (Mason, 1983). Recessing is achieved through a series of powerful adductions (valve closures) where water is ejected from the mantle cavity and lifts the shell at an angle to the seabed so that subsequent water jets blow a hollow into the sediment (Brand, 1991).

MobilitySwimming is generally limited to escape reactions. Experimental contact with different starfish species elicited distinct, energy adaptive types of response from Pecten maximus. Full swimming response was initiated only by extracts of Asterias rubens and Astropecten irregularis which prey on molluscs, while limited jumping or valve-closing responses were induced by non-predatory starfish (Thomas & Gruffydd, 1971).

Pecten maximus is capable of swimming by rapidly clapping the valves and expelling the water on either side of the dorsal hinge so that the scallop moves with the curved edge of the shell foremost (Thomas & Gruffydd, 1971). Jumping is achieved through the gradual relaxation of the adductor muscle followed by the rapid opening and closing of valves, which jump the scallop hinge forward (Thomas & Gruffydd, 1971).

Size and growthSpecimens of up to 21 cm have been recorded, although this is exceptional and the size range of scallops caught commercially is usually between 10 and 16 cm (Mason, 1983). Scallop shells bear distinct and concentric annual growth rings. The shells also bear numerous regularly occurring concentric striae 0.1-0.3 mm apart which are also used to age the scallops (Mason, 1957). Minchin (2003) stated that it took between three and six years to attain 11 cm in shell length. The Minimum Landing Size (MLS) for this species in Britain and Ireland is 10-11 cm (depending on area) and growth to this size is usually achieved within four years (Brand et al., 1991).Growth rate can be affected by several factors including salinity, temperature, competition, water depth and food supply. For example, Laing (2002) found that the growth rate of spat grown at 13-21 °C was significantly lower at 26 psu than at 28-30 psu. Mason (1957) found that specimens from inshore, shallower waters typically displayed higher growth rates and maximum sizes than those from deeper waters. Even differences in growth rate between different grounds have been reported (Mason, 1983). Growth in Pecten maximus slows down or stops altogether in the winter, starts again in spring and continues through summer when it is most active. Growth also becomes slower in older individuals and consequently the growth rings become closer together and difficult to distinguish (Mason, 1957). In contrast to many other studies on bivalves, Beaumont et al. (1985) found no association between heterozygosity and size in this species, i.e. genetic factors are relatively unimportant compared to environmental controls on growth. However, they also suggested that genetic factors may be more important during the larval stage.

EyesEmbedded among the bases of the sensory tentacles around the edge of the mantle are numerous tiny eyes (Mason, 1983). The eyes are a blue green colour no more than ca 1.5 mm in diameter. The eyes bear a superficial resemblance to the camera eyes of vertebrates and have a highly specialized retina (Wilkens, 1991). Light has both inhibitory and excitatory effects and scallops will swim, orient themselves or close their shell in response to shadows or movement (Wilkens, 1991).

Public healthCampbell et al. (2001) reported that, in July 1999, the Amnesic Shellfish Poisoning toxin, Domoic Acid (DA), was found in Pecten maximus at levels exceeding the regulatory limit of 20 µg DA / gram across large areas of northern and western Scotland. The risk of human illness resulting from consuming toxic scallops is, according to Shumway & Cembella (1993, cited in Campbell et al., 2001), a significant threat to both public health and the shellfish industry.

Water flow and wave exposurePecten maximus tend to be most abundant just inside or just away from areas of strong currents (Mason, 1983). Gibson (1956) found that scallops living in sheltered areas grew faster than those on wave exposed beds and suggested that this was because the feeding apparatus become overwhelmed by particulate matter in the highly wave exposed areas. It is also possible that the delicate processes of larval settlement and byssal attachment would be disturbed in strong currents (Brand, 1991).

Substratum The areas with highest abundance and the fastest growth rates of scallops are usually in areas with little mud (Brand, 1991). Gruffydd (1974) found that the maximum shell size of Pecten maximus from the north Irish Sea was significantly negatively correlated with increasing mud content in the sediment.

Aggregation and population subdivisionAdult scallops have a limited mobility and rely on the dispersal of larvae in terms of geographic distribution (Brand, 1991). The extent of this distribution will in turn be affected by factors including local hydrographic regimes and the survival of larvae. Consequently, all scallops have an aggregated distribution within their geographic range and the major fishing grounds are generally widely separated so much so that respective environmental conditions produce marked differences in population parameters (Brand, 1991).

However, in terms of genetic differences, two principle genetic population studies of Pecten maximus (Beaumont et al., 1993; Wilding et al., 1998) have failed to identify any evidence of sub-population structure (Beaumont , 2005). Wilding et al. (1999) found that the population of Pecten maximus from Mulroy Bay was more similar to Pecten jacobaeus than it was to other Pecten maximus populations, implying that this population is genetically distinct from others. This genetic isolation is thought to arise as a result of the enclosed nature of Mulroy Bay which probably means that the population is sustained through self-recruitment (Beaumont, 2005).

Reach first maturity at 2 years and full maturity at 3-5 years.Life SpanMason (1983) reported a scallop with 18 growth rings although he stated that beyond the ninth or tenth ring, they are hard to distinguish and so attract some uncertainty in terms of age. Minchin (2003) states that the maximum age for Pecten maximus is about 22 years. In reality however, especially in heavily fished areas, the average age / size is reduced and those caught commercially rarely exceed 16 cm (Minchin, 2003). The rate of natural mortality is low at 10-15 % for adult Pecten maximus (Rees & Dare, 1993).SpawningThe gametogenic cycle is highly variable and the timing of spawning may be influenced by both internal and external factors such as age and temperature respectively (Barber & Blake, 1991). Ansell et al. (1991) provide an excellent review of work done by several authors on populations of Pecten maximus in the Bay of Brest and the Bay of St Brieuc in France including work involving the transplantation of some individuals into different populations. They noted that differences in spawning cycles between populations reflect not only differences in their responses to local environmental variables but are also a consequence of genetic adaptation. In general, mature scallops spawn over the summer months from April or May to September. Estimates of gamete emission range from 15 - 21 million oocytes per emission for a three year old (Le Pennec et al., 2003). A bi-modal spawning pattern has been reported by several authors in different areas. In Manx waters for instance, Mason (1983) found most of the adults spawned partially in the 'spring spawning' in April or May and then more fully in an 'autumn spawning' event in late August. He also found that the virgins (scallops that have not spawned previously) and juveniles (those between their first and second spawns) only had one major spawning in autumn. Spawning is followed by a period of recovery of the gonad before the next spawn. Gibson (1956) found a similar bi-modal spawning in Bere Island sound (Ireland) but here the spring spawn was reported to be the most significant of the two. In the same study (Gibson, 1956), the Bantry Bay area scallops matured up to six weeks earlier than the Connemara area further north. Fertilization is external and either sperm or eggs can be exuded first (Mason, 1983).Dispersal Dispersal potential in Pecten maximus is high given that the length of the pelagic larval stage exceeds one month. In addition, Beaumont & Barnes (1992) have observed 'byssus drifting' in vitro which would provide a possible mechanism for the secondary dispersal of post-larval stages (spat). Some spat were observed to detach from the byssus thread and the subsequent production of a long and fine drifting thread slowed the decent of the spat thereby increasing the potential for dispersal (Beaumont & Barnes, 1992). Thouzeau & Lehay (1988, cited in Le Pennec et al., 2003) determined that Pecten maximus larvae could travel 10-40 km in 18 days due to tidal currents. However, Sinclair et al. (1985) hypothesized that using vertical migrations, larvae may be able to maintain their location within the confined of the scallop bed and that many aggregations are self-sustaining. Wilding et al. (1999) found that Pecten maximus from Mulroy Bay were genetically distinct to other populations. This genetic isolation is thought to arise as a result of the enclosed nature of Mulroy Bay which probably means that the population is sustained through self-recruitment (Beaumont, 2005).See General (larval) information for details of larval development and settlement.